As for "buckets to put tracking information into" why bother relying on "buckets" on the client which may or may not exist, are limited in size, may change or be emptied at any time, etc, when you can buy as many "buckets" as you want server-side and store virtually unlimited data about them?
Mainly, caching and offline access. When I access Gmail from my Android phone, I can read my e-mail offline, and browsing my inbox is instantly responsive even if my connection is very slow. This is because it just caches my whole inbox, any other tags I tell it to, and all mail from the last few days. If the Gmail website did that, which it can using localStorage, I wouldn't have to wait ten seconds sometimes to flip to the next e-mail while it retrieves it from the server.